Welcome,
We sail our O911S in lighter air on the Chesapeake Bay, so I haven't noticed too much weather helm, but then we haven't really pushed the boat in full crewed racing conditions either. That said, from previous discussions, most owners have been able to minimize the weather helm by adjusting the mast rake closer to vertical, putting mast forward at the partners to allow for more prebend, and tune the rig to the conditions you sail. I have not tried any of these suggestions, so YMMV. There are also some differences in how the Pacific built boats and the Ericson built boats respond to these changes.
As far as sails, what condition are yours? The ability to flatten them out will help with weather helm as well. If they are stretched and you can't flatten them, a new set will also help.
As a last thought, some owners have put a deeper Schumacher designed rudder on the boat which also helps in controlling the extra helm.
Another good source for O911 information is the mailing list. It's pretty much a ghost town these days but there is some decent information archived there. http://www.sailpix.com/email/olson911/
I'm sure that others will pop on here and ask you to post pictures of your boat in your user gallery, so please do that too.
Thanks for the information. Suspect it might be the sails, we are looking around for new ones, will update if it solves the problem.